Rotor imbalance

A friend of mine says he's seen a helicopter loop the loop at an air show. Is it really possible? And if it is, how? (Continued)

• The above photo is proof that looping the loop is possible in a helicopter. It shows an army Westland Lynx AH7 pulling over the top of a loop. You can see by the shadows that the helicopter is actually inverted. It was a spectacular sight to see and, as explained by your previous correspondents (The Last Word, 17 September), not impossible.